1. Field of the Invention
The present invention relates to saponified products of ethylene-vinyl acetate copolymer (referred to as EVOH hereinafter) compositions, which are superior in thermodrawability and in gas barrier properties, and which do not exhibit pinholes, cracks, localized areas of uneven stretching and the like, upon thermodrawing, and multilayered structures utilizing the same, especially thermodrawn, more particularly high speed thermodrawn multilayered structures.
2. Description of the Prior Art
EVOH resin is well known for its effectiveness as a food packaging film, especially for food requiring oxygen barrier packagings and other articles wherein the fragrance of the article must be retained. An EVOH single layer film is not tough enough and has insufficient barrier properties to moisture and water.
In order to overcome the foregoing disadvantages, EVOH is often used in combination with thermoplastic resins, such as, polypropylene, polystyrene and the like; and heat sealing layers, such as, ionomers, ethylene-vinyl acetate copolymers and the like; in the form of multilayered laminate structures.
In fabricated products, such as containers, formed from multilayered structures produced by various methods (films, sheets, parisons and the like), especially those drawn and formed below the melting point of EVOH, the EVOH layer exhibits many small voids, cracks, and localized areas of uneven stretching, such that the oxygen barrier properties of the formed containers are greatly reduced. Consequently, the formed containers cannot be used as packagings for food because of their poor appearance.
Several methods have been proposed for preventing pinholes, cracks and the like during the thermodrawing of multilayered structures containing EVOH. Japanese Patent Laid-open Nos. 88,067/1978 and 20345/1984 disclose the addition of various plasticizers to EVOH, and Japanese Patent Laid-open Nos. 141,785/1977, 154,755/1983 and 36,412/1983, teach blending of the EVOH resin with a polyamide resin. None of these methods have satisfactorily resolved these problems. Specifically, the method employing the addition of plasticizers containing hydroxyl groups, aromatic sulfonamides and the like, cannot be put to practical use because the addition of the plasticizers in the desired range of 10 to 20 weight percent with respect to the EVOH greatly reduces the gas barrier properties, and also decreases the adhesive strength between the EVOH layer and other plastic resin layers, presumably due to bleeding of the plasticizer.
Methods which comprise blending polyamide resins with EVOH in order to impart flexibility thereto and improve the fabricability are well known and many patent applications have been filed. (Japanese Patent Publication Nos. 24,277/1969, 24,813/1985, and 38,897/1984, Japanese Patent Laid-open Nos. 129,035/1983 and 36,412/1983.) However, a blend of polyamides capable of improving high speed thermodrawability cannot be put to practical use because a substantial amount of gels result in the formed articles presumably due to the high reactivity of the polyamides with the EVOH and substantial tinting (discoloring) results. Blends of polyamide with EVOH exhibiting less tinting (discoloration) are also known and produce esthetically pleasing formed articles without any apparent cracks, pinholes and localized areas of uneven stretching at low speed thermodrawing, but exhibit a large variability of observed values of gas barrier properties presumably due to minute pinholes, presumably because of the low compatibility of polyamides with EVOH. Furthermore, the recent speedup of thermodrawing machines causes increased scatter in the observed values for the gas barrier properties and results in decreased reliability as a gas barrier container.
The recent requirement for prolonged preservation of packaged food demands packaging exhibiting increased gas barrier properties. Thickening of the EVOH layer to meet these requirements tends to produce very small pinholes, cracks, localized areas of uneven stretching and the like as compared with the use of a relatively thin EVOH layer in thermodrawing. The thickening of the EVOH layer also results in increased variability in observed values for the gas barrier properties, which results in decreased reliability as a gas barrier container.
There is a tendency to blend scraps, such as trimmings from the formation process, containers and the like, with the thermoelastic resins in order to decrease product costs. In this case, the thermoplastic resin layer blended with the recovery plastic resins tends to exhibit localized areas of uneven stretching, cracks and the like, and becomes poor in appearance.
An object of the present invention is to produce an EVOH composition exhibiting excellent forming characteristics, superior gas barrier properties and high reliability as a gas barrier container (with less variability in observed values for the gas barrier properties) without pinholes, cracks, localized areas of uneven stretching and the like, at high speed thermodrawing.